1. Field of the Invention
The present invention relates generally to earth anchors and, more particularly, to an earth anchor having a load-bearing helix shaped to compensate for relatively high loading stresses which occur adjacent a trailing edge of the helix during installation and use of the anchor.
2. Discussion of the Prior Art
It is known, e.g. from U.S. Pat. No. 4,316,350, issued to Watson on Feb. 22,1982to provide an anchor with a load-bearing element in the form of an anchoring spiral which extends radially outward from a central body of the anchor by an amount which varies in the circumferential direction of the spiral between a lead point of the anchor and a trailing segment of the spiral.
In such constructions, it is further known to provide the spiral with a thickness which is constant across the radial extent of the spiral, but which increases in the circumferential direction of the spiral between a minimum thickness adjacent the lead point of the anchor and a maximum thickness at a trailing point of the spiral.
Although it is possible to increase the strength of an anchor by increasing the thickness of the load-bearing element in such a uniform manner, the use of a thickened element increases the weight of the anchor as well as the amount of material required to construct it. Accordingly, it would be beneficial to construct an anchor having a load-bearing element which is thickened only in the region where the load experienced by the anchor is at a maximum, while providing a reduced thickness in areas where loads of smaller magnitude are carried.
Earth anchors of conventional type are typically used to provide anchoring against a tensile load such as a utility pole or the like which is to be supported above the ground and retained against falling in a direction away from the anchor. When employed in this manner, an anchor is generally subjected to a tensile load which is believed to exert a substantially uniform pressure on the upper surface of the load-bearing element, i.e. the surface facing the object exerting the load on the anchor.
In the past, it has been a belief by some practitioners in the art of designing earth anchors that when such a uniform pressure is experienced by the upper surface of an earth anchor having a helix which extends around almost the entire circumference of the anchor, the location on the load-bearing element of highest stress is or should be immediately opposite the leading and trailing edge of the load-bearing element.
This theory is based upon consideration of the effect of such a uniform pressure on a generally C-shaped, annular flat plate having a radial cut-out portion extending completely through the plate. When such a plate is attached to an interior hub and a surface of the plate is loaded with a uniform pressure, failure of the plate would be expected to occur along a radial line located immediately opposite the radial cut since it is along this line that the least surface area of the plate exists to resist the load.
However, the present inventors have found that the predicted effect of failure occurring opposite the trailing or leading edge does not occur, and that by properly locating the actual point of high load stress on the load-bearing helix, an anchor may be locally strengthened to withstand a desired predetermined load limit.